Here, we consider entanglement measures to be nonnegative real functions \(\mathcal {E}(\rho )\)  such that [1]}, [2]}: i) \(\mathcal {E}(\sigma )=0\)  for all separable states \(\sigma \) ; and ii) \(\mathcal {E}(\rho )\)  does not increase on average under local operations and classical communication (LOCC)We note that this differs from the convention in, e.g., [2]}, where such quantities are referred to as `monotones'..
Many inequivalent measures can be defined, which result in different orderings of the entangled states. For this reason, it is usually favorable to consider measures that are associated to questions of practical relevance, such that they inherit a concrete meaning.
